الفهرس 
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Abstract
In the present study the temporal and spatial evolution of the plasma, produced by interaction of Q - switched Nd: Y AG laser pulses at 532 nm with pure aluminum target at different pressures (10-5 torr and atmospheric pressure), are investigated via optical emission spectroscopy (OES). Comparison of the temporal spectra taken at different distances from the target surface allows discussing the fundamental main processes that must be taken into account for the analysis of this kind of plasma. Analysis of these spectra revealed that at the early stages of the plasma evolution (at short delay times and close to the target) the spectra are dominated by continuum emission, due to mechanisms involving free electrons (inverse bremsstrahlung, radiative recombination, and photoionization). At this stage we can not obtain any information about atoms and ions. Some tens of nanoseconds after the laser pulse is over, the atomic and ionic lines rise up from the continuum. At this moment, the electron density is still very high and the Al I and Al II spectral lines are broadened by Stark effect due to the electron collisions. We can note that the maximum intensity of the spectral lines is reached when the core of the expanding plasma passes in front of the optical collection system. Depending on the observation distance and ambient gas pressure the first stage in temporal distribution of the spectra shows a regime where the ionic lines are more intense than the atomic lines, while on the tail of the temporal distribution-corresponding to the colder part of the plasma - the ionic lines disappear. When comparing the emission of species in vacuum and atmospheric pressure it is evident that for 0 < d < 1.5 mm the emission intensity has the same general behavior in the two environments, showing a maximum at a distance of 0.5 < d < 1mm from the target surface followed by a decay tail. The determination of the mean expansion velocity can be achieved by measuring the ionic emission temporal profiles (the so called Time of Flight (TOF) profiles). Ionic lines have been preferred with respect to the atomic ones because the Al I TOF can present an additional delay due to recombination mechanisms. In the range of distances investigated in this work, under vacuum condition the velocity of the laser induced plasma (LIP) increases with the distance. At atmospheric pressure, on the contrary, the LIP velocity immediately decreases with the distance, denoting that the deceleration of LIP, due to the collisions of the air particles with the front head of the plasma, occurring at very short distances from the target surface. It is clear that, changing the ambient pressure from vacuum (10-5 torr) to atmospheric pressure, a different dynamic of expansion takes place. The value of the electron density at atmospheric pressure is higher than in vacuum due to the electron-impact and confinement processes. The temporal evolution of Ne is found to diminish exponentially with time and then level off. The fast decay rate of electron density can be attributed to the plasma expansion, while the slowing and leveling off at longer times are most probably due to recombination. The temporal evolution of electron number density has been used for the estimation of the three-body recombination rate constant. The recombination time can be estimated by the values of the rate constant of recombination process, using the relation tree = (Ne 2 kreeyJ which gives a value of tree ~ 10-7 - 10-6 s. We can then conclude that t ion > t ree ~ t exp As a consequence of the fast expansion of LIP, the characteristic time of expansion is less than the corresponding time for establishment of Saha balance so the plume expands in quasi-equilibrium state and deviation from equilibrium has a recombination character. The excitation temperature of the LIP was estimated from the emission intensity of the atomic lines (AI I) and ionic lines (AI II). The variation of the temperature of the neutral species is almost insignificant in time, as it changes from 12000 to 8000 K, whereas the temperature of the ionized species decreased from 20000 to 11000 K. The observed difference between ionic and atomic temperatures is explained as the effect of radiative processes. The quasi-equilibrium state of the laser-induced plasma has been established due to the failure of Saha balance equation.